The plastic is moldable, and has a consistency of soggy cheese (I certainly hope I never encounter cheese that's anything like this!). When all is said and done, it should take you about 10-15 minutes to make the plastic (less if you make a small amount), 10 minutes of cleanup, plus 2+ days to allow the plastic to dry. As always, your milage may vary.
As for the final product's strength and whatnot, I would classify it as "okay." If you roll it thin (as I did in this instructable), it can easily be snapped in half, though it will probably survive a small drop on to carpet. Thicker pieces seem to be more resilient against average abuses - no problem dropping on to carpet, and if it's thick enough, you wouldn't be able to snap it in half. A blow from a hammer or other such object would quickly shatter it, though.
All in all, I would say this is mostly useful for folks looking for a fast and dirt cheap alternative to much better products available to the average joe. It's definitely not for something that will be handled on a daily basis - good enough for light decor, not much else.
This is my first Instructable, so bear with. Constructive criticism is always welcome!
Step 1: Ingredients & Utensils
Larger Pot (needs to be the same size as the other, or larger).
Spoon, preferably plastic or metal
Strainer or Colander, the finer the better
Stove or other item with which to simmer milk.
Paper Towels, lots and lots of paper towels
Handy, but not necessarily essential, items:
Wax Paper (good for rolling and shaping on. Water and other liquids bead up on it for easy clean-up; also non-stick with the casein)
Aluminum Foil (good for certain types of molds)
Rolling pin (if you're going to make a flat sheet of plastic)
Before you begin making the plastic, it's important to know what you're intending to make. By knowing what you're final goal is, you can make the preparations for the mold before you have a sloppy wad of plastic on your counter.
For this Instructable, I am making a curved piece that will fit on the front bezel of my computer case. Since this is beyond the scope of this particular instructable, I'm going to gloss over what I'm using. My mold is going to be the front bezel of the computer case, covered with aluminum foil. The plastic will dry nicely on top of it, and the aluminum won't shrivel or wrinkle when it gets wet as the plastic cures.
How much milk and vinegar will you need? The basic ratio that I follow is 1 TBSP of vinegar for every cup of milk. (16:1 ratio, milk to vinegar)
1 cup of milk will produce a puck of plastic about 2" in diameter, 1/8" thick.
To cover my case bezel, which is about 6 inches wide and 8 inches tall, I am going to use 8 cups of milk (and therefore 8 Tbsp of vinegar).